1. Field of the Invention
The present invention relates to a miniature motor designed for use as a driving motor in a fixed magnetic disk unit for office automation equipment and more particularly to the construction of a stator housing and a rotor of the miniature motor.
2. Prior Art
Miniature DC brushless motors are generally employed as driving motors for fixed magnetic disk units, and the construction of a conventional DC brushless motor is shown in FIG. 1. As shown in FIG. 1, a stator housing 1 of a motor is simultaneously used as a frame for mounting the motor and a stator 2 is fitted to the outer periphery of a boss part of the stator housing 1, the stator 2 being formed by winding an armature winding 4 on a stator core 3. On the other hand, a rotor comprises a rotary shaft 7 supported at two places via bearings 5, 6 axially fitted in a row onto the inner peripheral side of a central boss part 1a in the stator housing 1; a dished rotor yoke 8 of magnetic material connected to one end of the rotary shaft 7; and a magnet 9 installed opposite to the outer peripheral face of the stator 2 on the inner peripheral face of an outer peripheral rim part 8a of the rotor yoke 8. Moreover, a hub 10 for being loaded with a magnetic disk (not shown) is installed at the other end of the rotary shaft 7 which is passed through the stator housing 1. A magnetic shielding plate 11 for magnetically shielding the magnetic disk is attached face-to-face with one side of the stator housing 1 and an electric circuit required on the part of the stator is formed with the magnetic shielding plate 11 as a circuit board. Further, as shown in FIG. 1 a grounding ball 12 in contact with a chassis (not shown) is fixed into the end surface of the rotary shaft 7 and a compression coil spring 13 is held between the outer race of the radial ball bearing 6 and the stator housing 1. The compression coil spring 13 is responsible for holding the bearing 6 stable and displaceable during axial expansion and contraction of the rotor shaft due to temperature changes.
Referring to FIG. 2, the stator housing 1 of the aforesaid motor will be described further in detail. The stator housing 1 made of diecast aluminum has the magnetic shielding plate 11 bonded thereto with an adhesive agent. The stator 2 (not shown) is fitted to the outer periphery of the boss part 1a and the bearing 5 is fitted into the opening at the stator housing's one end on the inner peripheral side of the boss part 1a, whereas the bearing 6 and the coil spring 13 are fitted into the stator housing's opening from the other end. Then a stop ring 14 is used to fix the connection between the bearing 5 and the rotary shaft 7 when the rotary shaft 7 is inserted as shown in FIG. 1, completing the assembly. The outer race of the bearing 6 is supported across the stator housing 1 in such a manner as to be stopped from turning via a slide key, spline or the like.
Referring to FIG. 3, the construction of the rotor will be described further in detail. The dished rotary yoke 8 including the ringlike rim part 8a and a discoidal spider part 8b is first made by cutting or forging steel. The magnet 9 is installed on the inner peripheral side of the rim part 8a thereof. The combination of the rotary yoke 8 and the magnet 9 is then coupled to the rotary shaft 7 with an adhesive agent to form the rotor.
As set forth above, the stator housing 1 is made of diecast aluminum, and consequently a shrinkage void resulting from swallowing up air during the aluminum diecasting process tends to be produced in the layer thereof. Although the shrinkage void thus produced in the layer of the housing does not particularly affect its mechanical strength, the air is allowed to leak from the motor through the shrinkage void to the magnetic disk and dust penetrating in company with the air leakage from the outside may stick to the magnetic disk. Moreover, the diecast product cannot be subjected to accurate machining and needs more accurate finish machining for bearings and the like to be incorporated. In addition, a number of man-days are required to incorporate the magnetic shielding plate 11, the bearings 5, 6, etc. into the housing 1 thereby contributing to boosting motor production costs.
The aforesaid metal rotor is disadvantageous in that, because it is heavy and has the inertia moment increasing, greater starting torque essential to a driving motor for a fixed magnetic disk unit in particular is hardly unavoidable. In addition, the generation of noise is increased as the motor is operated. The disadvantages further include high production costs resulting from a number of man-days required for manufacturing, and assembling parts, including coupling the rotary shaft to the stator housing, into the rotor yoke.